1. Field of the Invention
This invention relates generally to a method for determining a rate of application of nitrogen fertilizer. More particularly, but not by way of limitation, the present invention relates a method for determining the existing nitrogen available for a plant and predicting the amount of nitrogen fertilizer to required to achieve a predicted yield potential.
2. Background
Presently, there is a need for a convenient method to determine the amount of fertilizer required to maximize the yield of a particular crop. While soil samples may be analyzed to determine the soil condition, the process is neither convenient nor is it conducive to advanced farming techniques such as precision farming.
xe2x80x9cPrecision farmingxe2x80x9d is a term used to describe the management of intrafield variations in soil and crop conditions. xe2x80x9cSite specific farmingxe2x80x9d, xe2x80x9cprescription farmingxe2x80x9d, and xe2x80x9cvariable rate application technologyxe2x80x9d are sometimes used synonymously with precision farming to describe the tailoring of soil and crop management to the conditions at discrete, usually contiguous, locations throughout a field. The size of each location depends on a variety of factors, such as the type of operation performed, the type of equipment used, the resolution of the equipment, as well as a host of other factors. Generally speaking, the smaller the location size, the greater the benefits of precision farming, at least down to approximately one square meter.
Typical precision farming techniques include: varying the planting density of individual plants based on the ability of the soil to support growth of the plants; and the selective application of farming products such as herbicides, insecticides, and, of particular interest, fertilizer.
In contrast, the most common farming practice is to apply a product to an entire field at a constant rate of application. The rate of application is selected to maximize crop yield over the entire field. Unfortunately, it would be the exception rather than the rule that all areas of a field have consistent soil conditions and consistent crop conditions. Accordingly, this practice typically results in over application of product over a portion of the field, which wastes money and may actually reduce crop yield, while also resulting in under application of product over other portions of the field, which may also reduce crop yield.
Perhaps even a greater problem with the conventional method is the potential to damage the environment through the over application of chemicals. Excess chemicals, indiscriminately applied to a field, ultimately find their way into the atmosphere, ponds, streams, rivers, and even the aquifer. These chemicals pose a serious threat to water sources, often killing marine life, causing severe increases in algae growth, leading to eutrophication, and contaminating potable water supplies.
Thus it can be seen that there are at least three advantages to implementing precision farming practices. First, precision farming has the potential to increase crop yields, which will result in greater profits for the farmer. Second, precision farming may lower the application rates of seeds, herbicides pesticides, and fertilizer, reducing a farmer""s expense in producing a crop. Finally, precision farming will protect the environment by reducing the amount of excess chemicals applied to a field, which may ultimately end up in a pond, stream, river, and/or other water source.
Predominately, precision farming is accomplished by either: 1) storing a prescription map of a field wherein predetermined application rates for each location are stored for later use; or 2) by setting application rates based on real-time measurements of crop and/or soil conditions. In the first method, a global positioning system (GPS) receiver, or its equivalent, is placed on a vehicle. As the vehicle moves through the field, application rates taken from the prescription map are used to adjust variable rate application devices such as spray nozzles. A number of difficulties are associated with the use of such a system, for example: due to the offset between the GPS receiver and the application device, the system must know the exact attitude of the vehicle in order to calculate the precise location of each application device, making it difficult to achieve a desirable location size; soil and plant conditions must be determined and a prescription developed and input prior to entering the field; and resolving a position with the requisite degree of accuracy requires relatively expensive equipment.
In the latter method, a sensor is used to detect particular soil and plant conditions as the application equipment is driven through the field. The output of the sensor is then used to calculate application rates and adjust a variable rate application device in real time. Since the physical relationship between the sensor and the application device is fixed, the problems associated with positional based systems (i.e., GPS) are overcome. In addition, the need to collect data prior to entering the field is eliminated, as is the need for a prescription map.
With either technique, there is a need to sense the soil and/or crop conditions in order to determine a rate of application of a given farm product. With regard to soil analysis, attempting to analyze the soil condition by way of a soil sample at each site would be time consuming and the handling of individual samples would be a logistical nightmare. Even with in-field analysis, the task would be daunting, at best.
In the past, the measuring of plant reflectance has shown some promise for identifying specific growing conditions. The measurement of plant reflectance is noninvasive to growing crops, may be performed very quickly, and is exceptionally conducive to advanced farming techniques. Unfortunately, there has been no method to interpret such information to determine the application rate of fertilizer. An example of a device which uses reflectance for the selective application of herbicide is described in U.S. Pat. No. 5,585,626 issued to Beck et al.
Thus it is an object of the present invention to provide a convenient method for determining an application rate for the in-season application of fertilizer nitrogen, which is non-invasive to growing crops and is conducive to advanced farming techniques.
The present invention provides a method for determining in-season fertilizer nitrogen application based on predicted yield potential and a nutrient response index. In a preferred embodiment of the inventive method, an optical sensor is used to measure the reflectance of a target plant at one or more wavelengths of light and, based on known reflectance properties of the target, an output is provided which is indicative of the need for nitrogen. The inventive process, however, is not limited to nitrogen but rather is applicable to any crop nutrient whose projected need could be based on predicted uptake in the grain, derived from predicted yield.
Efficiency of site-specific fertilizer management is largely determined by how well small-scale spatial variability is managed and the time when fertilizers are applied. During the crop growing season (in-season), knowledge of yield potential is a key to successful variable rate fertilizer applications, particularly for topdress nitrogen. Maximum yield potential (xe2x80x9cYPMAXxe2x80x9d) is the theoretical maximum dry grain yield that could be produced per unit area when manageable yield factors are non-limiting in a specific growing season. In the inventive method, a normalized difference vegetation index (NDVI) is calculated from reflectance information gathered by scanning a plant. Once NDVI is determined, an in-season estimated yield (INSEY) index may be calculated by dividing NDVI by the number of growing days from planting, which is further used to determine potential yield (YP0), the predicted yield with no additional nitrogen fertilization. A response index (RINDVI) is calculated, which is a measure of the expected response of the crop to adequate levels of fertilizer. RINDVI is determined by sensing NDVI of plants in a plot receiving adequate but not excessive pre-plant nitrogen divided by the NDVI of plants receiving conventional management. Based on these measurements, the predicted yield which can be attained with added nitrogen (YPN), may be projected by the equation:
YPN=YP0*RINDVI
Through a series of calculations, YPN is used to determine the topdress fertilizer nitrogen requirement.
Virtually any method of measuring the reflectance of individual plants or small groups of plants will provide the desired results. However, preferred methods of measuring reflectance include: 1) the use of a passive sensor as described hereinbelow or 2) the use of an active sensor as described in co-pending U.S. patent application, Ser. No. 09/912,077 entitled xe2x80x9cOptical Spectral Reflectance Sensor and Controllerxe2x80x9d and filed contemporaneously herewith, which is incorporated herein by reference.
Typically, an active sensor comprises: a light emitter which provides one or more light sources, each source producing light at a specific wavelength; a modulator for modulating each light source at a particular frequency, a reflected light receiver for receiving, detecting, and discriminating each wavelength of light; a direct receiver for receiving light directly from each source; and a processor for gathering information from the receivers and processing such information to determine reflectance of a plant.
A passive sensor, on the other hand, includes up-looking detectors and down-looking detectors such that the actual reflectance of the target plant may be determined. The desired wavelengths of light are separated for use by both the up-looking and down-looking detectors using optical filters or the like.
In the preferred method, the reflectance properties of a target are known to vary based on the amount of nitrogen available to the plant. By measuring the reflected light, at particular wavelengths, preferably in the ranges of red and near infrared, and the intensity of the light source at the same wavelengths, it is possible to predict, with a reasonable degree of certainty, the expected crop yield with the present level of available nitrogen and the maximum crop yield if an ideal amount of nitrogen fertilizer is added. This information may be used in real time to control a variable rate applicator for applying a mid-growing season nitrogen fertilizer or, alternatively, used to develop a prescription map for later application of mid-growing season nitrogen fertilizer to a field. The location size in a site-specific application utilizing the present method is limited only by the resolution of the sensor and the resolution of the applicator.